Control for refrigeration apparatus



April 14, 1970' D. R. VAND ER MOLEN ETAL 3,505,323

- CONTROL FOR REFRIGERATION APPARATUS I Filed June 20, 1968 2Sheets-Sheet 1 FIG. I CONDENSER coMPREssoR 23 CQNTROL i A R 25COMPRESSOR ON A OFF FAN SPEED HIGH LOW I g I =TEMP. 1. l 1, i 1 l l I I46 coouws 1 CAPACITY m 44 I I) '45 c 1 /l i I I o v TEMP.

SELECTED TEMPERATURE INVENTORS DONALD RVANDER MOLEN DANA E.WH|TLOWvATTORNEYS.

FIGZ

April 14, 1970 'ID.-R. VANDER 'MOLEN E -x 1 0 8 7 CONTROL FORREFRIGERATION APPARATUS Filed June 20, 1968 2 sheets-sheaf 2 UnitedStates Patent 3 505,828 CONTROL FOR REFRIGERATION APPARATUS Donald R.Vander Molen, Stevensville, and Dana E. Whitlow, St. Joseph, Mich.,assignors to Whirlpool Corporation, a corporation of Delaware Filed June20, 1968, Ser. No. 738,609 Int. Cl. F25d 17/00 US. Cl. 62180 9 ClaimsABSTRACT OF THE DISCLOSURE An air conditioning control having a twostage temperature compensated transistor amplifier connected to athermistor temperature sensing bridge for'cycling the compressor on andoff. The speed of a fan for the evaporator is varied above a minimumspeed in accordance with temperature when the compressor is operating.

This invention relates to a control for: refrigeration apparatus, andmore particularly to a refrigeration control for regulating the speed ofa fan and the operation of a compressor unit.

A refrigeration apparatus, such as a conventional room type airconditioner, has an evaporator through which air is passed by a fan toeffect a heat exchange for cooling the air. A refrigerant fluid iscirculated through the evaporator by a compressor which is cycled on andoff by a control unit. The control unit is connected to a temperaturesensor arranged to initiate operation of the compressor when the roomair temperature rises above a preselected high value, and to discontinueoperation of the compressor when the room air temperature falls below apreselected low value.

It has been suggested that the speed of operation of the fan for theevaporator may be varied in proportion to temperature, independent ofthe operation of the compressor circuit to vary system capacity. Whilesuch a control represents an improvement over prior controls, it doesnot produce a system having the desired maximum efiiciency at varyingcontrol settings.

In accordance with the invention, control of the evaporator fan speed issynchronized with control of energization and deenergization of thecompressor unit, to produce a refrigeration system having improvedcooling and dehumidifying efliciency over a wide range of controlsettings. The speed of the fan is preferably increased subsequent to theenergization of the compressor to increase the cooling capacity of therefrigeration apparatus. A desired temperature is maintained by varyingthe speed of the fan and thus the system cooling capacity within apreselected controlled range. Since the compressor is continuouslyenergized when the control calls for cooling, the evaporator remainscool in order to condense water vapor and hence dehumidify the air.

A principal object of this invention is to provide an improvedrefrigeration apparatus control which synchronizes control of theoperation of the evaporator fan with control of the operation of thecompressor.

One of the features of the invention is'the provision of an airconditioner control in which evaporator fan speed is varied while thecompressor is operating. While the compressor'is deenergized, theevaporator fan is maintained at a constant minimum speed.

Another feature of this invention is the provision of an air conditionercontrol having a temperature sensor circuit for switching a compressoron and off. The temperature sensor circuit also controls the firingangle of a switching device connected between a fan motor and a sourceof alternating current.

Patented Apr. 14, 1970 Yet another feature of this invention is theprovision of an air conditioner control in which a conduction deviceconnected to a fan motor is controlled by a circuit including a voltagedivider for maintaining a desired minimum voltage. The circuit iscontrolled by the same temperature sensor which cycles a compressorbetween energized and deenergized states.

Still another feature of the invention is the provision of a control forrefrigeration apparatus employing temperature compensation forsemiconductor devices used in the control.

Other features and advantages of the invention will be apparent from thefollowing description of one embodiment taken in conjunction with theaccompanying drawings. Of the drawings:

FIGURE 1 is a block diagram of refrigeration apparatus controlled by theinvention;

FIGURE 2 is a schematic diagram of the control shown in block form inFIGURE 1; and

FIGURE 3 illustrates curves of the operating characteristics of thecontrol over a limited range of temperatures.

While an illustrative embodiment of the invention is shown in thedrawings and will be described in detail herein, the invention issusceptible of embodiment in many different forms and it should beunderstood that the present disclosure is to be considered as anexemplification of the principles of the invention and is not intendedto limit the invention to the embodiment illustrated. Throughout thespecification, values and type designations will be given for certain ofthe components in order to disclose a complete, operative embodiment ofthe invention. However, it should be understood that such values andtype designations are merely representative and are not critical unlessspecifically so stated. The scope of the invention will be pointed outin the anpended claims.

Turning to FIGURE 1, refrigeration apparatus in the form of an airconditioner 10, which may be of the room air conditioner type adaptedfor use in a window or through-the-wall installation, is controlled by aunit 11 constructed in accordance with the invention. The airconditioner 10 is enclosed in a suitable housing (not shown) and has anevaporator 13 with refrigerant conducting tubes (not illustrated) whichmay have heat transfer fins thereon through which room air is circulatedby means of a fan or blower 14. A compressor 16 is connected withevaporator 13 for circulating a refrigerant fluid through the evaporator13 and through a condenser 17 connected in a closed fluid path with acompressor 16 enclosed in a housing including a compressor motor. Therefrigerant line connecting condenser 17 to vaporator 13 includes theusual restrictor, illustrated in the form of a capillary 9. A second fan19 is disposed adjacent condenser 17 for circulating cooling airthereto. Fan 19 is located behind a transverse partition 20 whichseparates fan 19 and condenser 17 from fan 14 and evaporator 13 in theair conditioner housing.

Control unit 11 maintains room air at the temperature selected by theuser of the apparatus through control of the operation of bothcompressor motor 16 and a fan motor 22, which may be arranged to driveboth fans 14 and 19 as illustrated in FIGURE 1. It will be understoodthat fan 19 could also be operated by another control and another motorindependent of control 11 and motor 22. A temperature sensor illustratedin the form of a thermistor 23 is located in the path of air enteringevaporator 13 for sensing the temperature of room air drawn through theair conditioner by fan 14. When the sensed air temperature exceeds apredetermined value, control unit 11 energizes compressor motor 16. Theapparatus discussed above, except for the structure and operation ofcontrol unit 11 discussed in detail hereafter, is generally known andwill not be described in detail.

Turning now to FIGURE 2, control unit 11 is shown in schematic form. Apair of lines from a conventional AC power source (not illustrated)provide power through a transformer 50 to a DC supply circuit whichprovides operating power to a thermostat circuit 31, which includestemperature sensor 23, and a fan motor speed circuit 32. Thermostatcircuit 31 cycles compressor motor 16 between on and off states inaccordance with a set point temperature selected by adjusting a controlknob (not shown) associated with control unit 11, and the relationshipbetween set point temperature and the room temperature as monitored bysensor 23. Fan speed circuit 32, which is responsive to signals fromthermostat circuit 31 when the compressor is energized, passes avariable amount of AC current to fan motor 22 for varying the speed ofthe fans driven thereby. An understanding of the general operation ofcontrol unit 11 may be facilitated by referring to FIGURE 3, in whichcurves are illustrated of the operating characteristics of compressor 16(FIGURE 3A), fan speed circuit 32 (FIGURE 3B), and the overall system(FIG- URE 3C). The three curves have a common base which represents alimited range of temperatures near the temperature selected by the user.

In operation, thermostat circuit 31 turns the compressor on at a setpoint temperature indicated at 35, as seen in FIGURE 3A. The compressorremains on for all temperatures in excess of temperature 35, and willnot turn ofi until a lower temperature indicated at 37 is reached. Oncecompressor 16 is off, the room temperature as monitored by sensor 23must rise to the higher value indicated at 35 before the compressor willagain be turned on. In the embodiment of FIGURES 1-3, the temperaturediiterential between temperatures 35 and 37 is approximately 1 F.

Fan speed control circuit 32 normally maintains the fan at a low speedindicated at 39, FIGURE 3B, for all temperatures below the temperatureindicated at 35 at which the compressor is turned on. The operation offan speed control circuit 32 is synchronized with the operation ofthermostat circuit 31 so that when the compressor is turned on attemperature 35, the speed of the fan may be modulated by being increasedcontinuously over a range indicated at 40 until a maximum speedindicated at 41 is reached for all temperatures in excess of atemperature indicated at 42, higher than the set point temperatureindicated at 35. As shown in FIG- URE 3B, a temperature indicated at 36falls within the range of continuous speed control of the fan motor 22.If the air conditioner capacity has been properly matched with therequired cooling load, fan speed control 32 will respond to maintain theroom temperature at the temperature indicated at 36. The temperatureindicated at 36 may be called the control point temperature. Thetemperature differential between the temperature indicated at 42, atwhich maximum fan speed indicated at 41 occurs, and the temperatureindicated at 35, at which 'theicompressor is first energized, isapproximately 1 F.

The resulting cooling capacity of the air conditioner when operated inthe above manner can be seen in FIGURE 3C. The cooling capacity of theair conditioner has a constant value indicated at 44 when the compressoris on and the fan is driven at low speed. For temperatures in excess ofthe temperature indicated at 35, however, the increase in fan speed hasthe effect of increasing the cooling capacity of the air conditioner,along a curve 45, until a maximum cooling capacity indicated at 46 isreached which represents the compressor on and the fan operating atmaximum speed. The control unit is efiective to vary the fan speed tomaintain a room temperature at a control point temperature indicated at36 which falls within the variable cooling capacity curve of the airconditioner. Depending on the cooling load,

the control pointtemperature may not fall within the variable coolingcapacity range. Thus under high load conditions, the control pointtemperature may be in excess of the temperature indicated at 42.

By synchronizing the start of the fan speed modulation with the initialenergization of the compressor, and contlnuously varying fan speedbetween minimum and maxlmum values over a narrow range of temperatures,several important operating advantages are achieved. The full range offan speeds is available over a narrow temperature ditferential in orderto control more precisely the capacity necessary to maintain a selectedtemperature. Furthermore, the compressor is maintained on for amaxll'Illll'Il time, keeping the evaporator coils cool to condense watervapor and hence control the humidity of the air over a maximum period oftime. Also, when the compressor is first turned on, indicating sensingof a rising temperature, a variable rather than a fixed cooling capac-1ty is available to interrupt the sensed temperature rise. Furthermore,the air conditioner will always operate at the lowest fan speedconsistent with the cooling load and thus provide quieter operation.This is particularly advantageous for room air conditioners used forcooling bedrooms where the night cooling load is typically low and whereundue noise will interfere with sleeping. Also, legislation now in forceestablishing permissible noise levels for air conditioners sets lowerlevels for nighttime operation. Thus, it is desirable for the. airconditioner to be quieter in operation during the nighttime hours whileachieving the desired cooling capacity.

Returning to FIGURE 2, the control unit 11 which produces the aboveresults will now be described in detail. Power supply circuit 30consists of a power transformer 50 which drops the AC voltage inputthereto from power lines 25 to 24 volts, which is coupled to a full wavediode rectifier bridge 52. The output of bridge 52 is connected througha ohm resistor 53 to a 20 volt Zener diode 54 for clipping the full waveAC voltage. The voltage across Zener diode 54 is coupled directly to fanspeed circuit 32 and to a blocking diode 56 and a microfarad smoothingcapacitor 57 for powering thermostat circuit 31.

Temperature sensor 23 in thermostat circuit 31 is a negative temperaturecoefficient thermistor, having 1,000 ohm resistance at 77 F., which isconnected in a bridge type circuit across DC output capacitor 57. Oneside of the bridge consists of a 10 kilohm potentiometer 60, a 4.7kilohm resistor 61, thermistor 23, and a diode 6-3 connected in seriesacross the DC supply. The other side of the bridge consists of a 10kilohm resistor 65 and a 1 kilohm resistor 66 connected in series acrossthe DC supply. The output from the thermistor bridge is available at ajunction 68 between resistor 61 and thermistor 23, and a second junction69 between resistor 65 and resistor 66. The resistance of potentiometer60 is varied by the user of the air conditioning system to select thedesired set point temperature, as may be indicated by a suitablycalibrated control knob (not illustrated) associated with control unit11.

A two stage transistor amplifier using NPN transistors 71 and 72 isconnected between the thermistor bridge and a relay coil 73 forenergizing compressor motor 16. Transistor 71 has its base directlyconnected to junction 68. The collector of transistor 71 is directlyconnected to the base of transistor 72, and is connected to the DC powersupply through a 100 kilohm resistor 75. A 2 kilohm potentiometer 76connects the emitter of transistor 71 to the junction 69 on the oppositeside of the thermistor bridge.

The collector of transistor 72 is connected through relay coil 73 withthe DC power source. Relay coil 73 has a 2.0 kilohm resistance with a4.9 milliampere pullin and a 2.9 milliampere drop-out rating. When thevalue of pull-in current is reached or exceeded, a normally open contactswitch 73-1 is closed to energize compressor motor 16. The emitter oftransistor 72 is connected to a voltage divider consisting of a 3.3kilohm resistor 77 and a 680 ohm resistor 78 connected in series acrossthe DC supply, with the junction therebetween being directly connectedto the emitter of transistor 72.

Compressor motor 16 is connected across the AC power lines 25 through anormally nonconducting semiconductor bidirectional triode switch ortriac 80. Triode switch 80 is shunted byrelay contact 73-1 in serieswith a pair of resistors 81 and 82. The junction between resistors 81and 82 is connected to a gate terminal 83 of the triode switch.

In operation, transistor 71 is driven toward saturation attemperatures'below that selected by potentiometer 60. Transistor 71diverts base current away from transistor 72, tending to drivetransistor 72 more nonconductive as transistor 71 becomes moreconductive. As the temperature sensed by thermistor 23 increases, thethermistor resistanceand corresponding voltage drop decreases, drivingtransistor 71 towards cut-01f and increasing the conduction oftransistor 72. At the temperature indicated at 35 in FIGURE 3A,transistor 72 passes sufficient current to pull in relay 73, therebyclosing relay contact 73-1 and gating triac 80 into conduction. Triac 80thereupon passes both half cycles of AC power to compressor motor 16causing operation of the compressor. Since relay 73 has a smallerdrop-out current than that required for pull-in, compressor motor 16will not be turned off until a lower temperature 37 is sensed bytemperature sensor 23.

The operation of fan speed circuit 32 is synchronized with the operationof thermostat circuit 31. More particularly, a 25 kilohm resistor 90 isconnected in parallel with transistor 72 and resistor 78' so thatvoltage thereacross is approximately inversely proportional to theconduction of transistor 72. Resistor 90-is in the form of apotentiometer having a variable tap 91 connected to a ramp and pedestalcircuit 92 for controlling the firing angle of a second triac 93whichcontrols the current to fan motor 22. Ramp and pedestal circuit 92,to the extent described herein, operates in a known manner for circuitsof this type, and reference may be made to a standard semiconductorcontrol device manual for a more detailed description, as the GeneralElectric Company SCR Manual, third edition, 1964, pages 130 to 137.

Coils 100 and 100a, and capacitors 102 and 102a form a radio frequencyinterference filter for eliminating radio interference generated bytriac 93. Triac 93 is in series with fan motor 22 in order to controlthe amount of each half cycle of AC which is passed to the motor. Triac93 has a gate terminal 105 whch isconnected through a pulse transformer106 to a unijunction transistor (UJT) 107. UJT 107, in turn, is undercontrol of ramp and pedestal circuit 92 for controlling the firing angleof triode switch 93.

Ramp and pedestal circuit 92 includes an input NPN transistor'110 havingits base directly connected to tap 91 of potentiometer 90. The collectorof transistor 110 is connected through a 1.5 kilohm resistor 112 topower supply line 114 which is connected to Zener diode 54. Aspreviously explained, line 114 provides a clipped or regulated full waverectified DC voltage. The emitter of transistor 110 is connected througha one kiloohm resistor 116 to the opposite side of the DC power supplycircuit 30. UJT 107 has its emitter 107 connected to a diode 117 and toone side of a 0.33 microfarad capacitor'118, and its B1 terminal 107connected through the primary side of pulse transformer 106 to theopposite side of capacitor 118. UJT 107 is connected to power line 114through a 22 kilohm resistor 120 connected with emitter 107 and a 1.5kilohm resistor 122 connected with B terminal 107 When the voltageacross capacitor 118 reaches a predetermined level, UJT 107 fires todischarge the capacitor, through pulse transformer 106. After UJT 107 isturned off by the momentary drop of DC voltage on line 114, capacitor118 begins to charge again. The voltage across capacitor 118 increasesrapidly to a pedestal level through the low resistance of resistor 112,and thereafter increases more slowly along a ramp shaped curve as thecapacitor is charged through the relatively higher resistance ofresistor 120. Thus, the firing voltage of UJT 107 is reached in a twostep charging of capacitor 118. When the voltage reaches a predeterminedfraction of the total voltage across the terminals 107 and 107 UJT 107fires, discharging capacitor 118 through pulse transformer 106 andfiring triac 93.

As transistor 72 in thermostat circuit 31 is driven toward saturation,the voltage drop across resistor decreases, causing transistor to shuntless of the current passing through resistor 112, and hence raising thepedestal; This advances the time of firing of UJT 107 thereby gatingtriac 93 sooner in each half cycle.

As previously explained, the fan motor 22 is main- I tained at'aconstant low speed when the compressor is not energized. For thispurpose, a 1.5 kilohm resistor 130, and a variable potentiometer 131,having a maximum 5 kilohm resistance, are connected in series across theDC source. The junction between voltage dividing resistors and 131 isconnected through a diode 132 to the emitter 107 of UJT 107.

Capacitor 118 is charged to its initial pedestal level through resistor112 and/or resistor 130. Thus, although transistor 110 may shunt thecurrent available from resistor 112, capacitor 118 will rapidly chargeto a fixed pedestal level determinedby variable potentiometer 131,establishing a minimum speed for the fan motor 101. Accordingly,variable potentiometer 131 is adjusted to set the desired minimum speedfor the fan motor.

The tap 91 on potentiometer 90 is adjusted to establish the temperaturediiferential between the initiation of variable speed fan operation andthe initial energization of compressor motor 16. In FIGURE 3, theindicated temperature diiferential is zero, and fan motor speedincreases with any increase in temperature above the set pointtemperature indicated at 35 at which compressor 16 is energized.However, potentiometer 90 may be set to initiate fan motor speed controlat temperatures less than or in excess of the set point temperatureindicated at 35. If initiation of fan motor speed control is notconcurrent with compressor energization, it is preferred that fan speedcontrol be initiated at temperatures in excess of the set pointtemperature. In FIGURE 3B, the broken line 4001 indicates the speedchange curve when speed changes are initiated below the set pointtemperature indicated at 35, and the dotted line 40b indicates the speedchange curve when speed changes are initiated above the set pointtemperature indicated at 35. The effect on capacity of changing thepoint at which fan speed change is initiated is indicated in FIGURE 3C.The broken lines 45a show the below set point curve and the dotted lines45b the above set point curve. A comparison of the capacity curves willindicate the desirability of initiating fan speed changes at or aboveset point temperature indicated at 35. When fan speed changes occurbelow set point temperature indicated at 35, the air conditioner startsat an intermediate capacity, with a loss of modulation between theminimum capacity and the intermediate capacity when room temperature isrising. Full capacity modulation is available when the room temperatureis decreasing because the compressor otf temperature is less than thetemperature at which the fan motor 22 reaches minimum speed.

When the fan speed changes are initiated at temperatures in excess ofthe set point temperature, the range of temperature between compressorOE and full capacity is increased. This does not affect the capacitymodulation characteristics of the control but becomes undesirable from acomfort standpoint in that the control reacts more slowly to risingtemperatures and may allow room temperature to reach levels in excess of2 F. above the set point temperature. Some people are sensitive totemperature changes of as little as 1 F. and most people are sen sitiveto temperature changes of 35 F. Thus if the control is caused to reacttoo slowly to temperature riseabove set point temperature, some peoplewill be uncomfortable in the room being cooled by the air conditioner.

For a temperature differential of zero and with pull-in current flowingthrough relay 73, tap 91 is adjusted to produce the same pedestalvoltage at diode 117 as exists at diode 132. Thereafter, as the relaycurrent increases beyond the pull in value, due to an increasingtemperature, the pedestal voltage is increased in proportion totemperature, aligning the beginning of curve 40 of FIG- URE 3B with thetemperature indicated at 35 at which the compressor is first energized.It will be readily understood that tap 91 may be adjusted to providelarger or smaller voltages at diode 117 and thus change the temperatureat which initiation of fan motor speed changes occur. Once the tap 91has been set, thus establishing the temperature differential between setpoint temperature indicated at 35 and the temperature at which fan motorspeed change is initiated, the potentiometer 60 may be used to establishset point temperature indicated at 35 and synchronously the temperatureat which fan motor speed change is initiated. The differential betweenthese two temperatures will remain constant over the range of set pointtemperature settings permitted the user. In the illustrated embodimentof the invention the user can obtain any desired set point temperaturewithin a range of 60-90 F.

Regardless of the setting of minimum speed potentiometer 131, UJT 107 isforced to fire at the end of each half cycle due to the momentary dropto zero of supply voltage on line 114. As UJT 107 fires, capacitor 118is discharged; insuring uniform performance on all half cycles sincecapacitor 118 will always be charged from a fully discharged condition.

Control circuit 11 also includes temperature compensation for thesemiconductor devices used therein to insure repeatable performanceregardless of the air temperatures surrounding the components.Potentiometer 76, in series with the emitter of transistor 71, providestemperature stability by reducing the sensitivity of thermostat circuit31 to beta variations of transistor 71, as well as providing aconvenient adjustment of the sensitivity (deadband). As leakage currentsthrough the transistor vary with changes in temperature, the voltagedrop across potentiometer 76 is added as a feedback voltage to the fixedoperating voltage established by resistors 65 and 66. This feedbackvoltage may be in a positive or negative direction. Resistor 78 servesthe same purpose for transistor 72 as potentiometer 76 serves fortransistor 71, thereby increasing temperature stability.

Diode 63 is preferably located in heat transfer relation with transistor,71, as indicated by dashed lines 135. By using the same typesemiconductor material for diode 63 and transistor 71, the voltagechange in one device cancels the voltage change in the other fortemperature variations. With the temperature compensation describedherein, control unit 11 remains in calibration and synchronizationregardless of the heating of the components used therein.

While we have shown and described certain embodiments of our invention,it is to be understood that it is capable of many modifications.Changes, therefore, in the construction and arrangement may be madewithout departing from the spirit and scope of the invention as definedin the appended claims.

The embodiment of the invention in which an exclusive property orprivilege is claimed is defined as follows:

1. A control of refrigeration apparatus including an evaporator forchilling air, an evaporator fan for moving air over said evaporator, anevaporator fan motor for 8 driving said evaporator fan and a compressorfor circulating refrigerant to said evaporator, comprising:

first circuit means connected to said evaporator fan motor for operatingsaid evaporator fan motor at a first speed;

means for sensing temperature;

second circuit .means responsive to said means for sensing temperaturefor energizing said compressor when a predetermined temperature issensed by said means for sensing temperature; third circuit meansincluding said first circuit means,

connected to said second circuit means and responsive to said means forsensing temperature for varying the speed of said evaporator fan motorfrom said first speed in response to deviation of the temperature sensedby said means for sensing temperature from said predeterminedtemperature, thereby varying the cooling effect of said refrigerationapparatus by varying the rate of air flow moving over said evaporator;

said third circuit means further including a device switchable betweenconducting and nonconducting states under control of a gating signal, asource of alternating current for driving said fan motor, meansconnecting said device between said AC source and said fan motor, andfiring angle control means for with respect to a phase of saidalternating current to control the firing angle of said device inproportion to the sensed temperature deviation from said predeterminedtemperature; and

wherein said second circuit means includes a thermistor havingresistance proportional to temperature, current operated switching meansfor energizing said compressor when a predetermined pull-in current isapplied thereto, semiconductor means connected to said thermistor and tosaid current operated switching means for generating said pull-incurrent when said thermistor senses said predetermined temperature, saidthird circuit means includes an impedance device coupled to said secondcircuit means and having a voltage thereacross proportional to thecurrent through said current operated switching means, and said firingangle control means is connected between said impedance device and saidswitchable device to vary the time of occurrence of said gating signalin proportion to the voltage across said impedance device.

'2. The control of claim 1 wherein said second circuit means includes atemperature responsive voltage divider having in series a variableresistor and said thermistor, said semiconductor means including anoutput transistor having a current proportional to the voltage acrosssaid thermistor, said current operated switching means comprising arelay coil in series with said output transistor for actuating saidcompressor when pull-in current flows through said output transistor,and said firing angle control means includes a transistor responsive tothe voltage across said impedance device when current in excess ofpull-in current flows through said relay coil for varying the firingangle of said switching device.

3. The control of claim 2 wherein the transistor in said firing anglecontrol means is connected in a ramp and pedestal circuit, and saidfirst circuit means generates a minimum pedestal for maintaining saidfan at a minimum speed corresponding to said first speed condition.

4. The control of claim 1 wherein said second circuit means includesmeans temperature compensating said semiconductor means to changes inambient temperature.

5. The control of claim 4 wherein said temperature compensating meansincludes a semiconductor device connected in an electrical path withsaid thermistor, said device being formed of the same semiconductormaterial as said semiconductor means and disposed in heat transferassociation with said semiconductor means.

6. The control of claim 4 wherein said semiconductor means includes atleast one transistor having base, collector and emitter electrodes,means connecting said base electrode to said thermistor, a source of DCpower, means connecting said DC power source in an electrical pathincluding said collector and emitter electrodes for gen erating saidpull-in current, and said temperature compensating means includesresistive means located in said electrical path to reduce thesensitivity of said second circuit means.

7. The control of claim 1 wherein said second circuit means includesmeans coupling said AC source to said compressor when said predeterminedpull-in current is applied to said current operated switching means,said circuit means having no current path from said thermistor to saidAC source, thereby isolating said thermistor from said AC source.

8. The control of claim 7 wherein said circuit means solely includes airgaps with magnetic fields therein for signal transfer between saidcircuit means and the means connected to said AC source.

9. The control of claim 1 including a source of DC power, and meansconnecting said DC power source to said second circuit means forgenerating said pull-in current, thereby isolating said currentoperating switchin means from said AC source.

References Cited UNITED STATES PATENTS WILLIAM J. WYE, Primary 'ExaminerUS. Cl. X.R. 2-207; SIG-15? Patent No.

P0-1050 (s/ss) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONDated April 14, 1970 lnv nt fl Donald R. Vander Molen et a1 It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 7, line 73, in Claim 1, the word "of" should be --fOr-- Column 8,line 26, in Claim 1, after "for" should be said gating signal-- Column10, lines 6-7, in Claim 9, the word "switchin" Atteat:

should be -switching-- SIGNED AND SEMI-YD SEP8-1970 EdwardMFletcherJr.

Attesting Officer

